B&S MODEL ZZ 1930-40S

HI THERE EVERONE
im looking at a briggs and stratton model zz
it has milatry green paint remaining on most of the engine
it has cast iron cowlings ,block and *********cylinderhead **********
all the z, zz and zl ive found on the net are all side sparkplug with alloy heads with either smooth top or finned
THIS ONE IS CAST IRON HEAD WITH SMOOTH TOP AND TOP CENTER MOUNTED SPARKPLUG ?????
ANYONE HAVE ANY IDEAS WHY THIS ONE IS DIFFERENT IVE BEEN ON THE BRIGGS AND STRAT WEBSITES AND THEY ONLY LIST THE ALLOY SIDE PLUG TYPES AND ALL THE OTHER MODELS IVE LOOKED AT ON THE NET HAVE COMPLETLY DIFFERENT HEADS ON
HAS ANYONE ELSE GOT A Z ZZ OR ZL ******** ???????? WHAT IS THE VALUE ???????********
HOW RARE ARE THEY AS IVE NEVER SEEN ONE BEFORE
PLEASE EMAIL WITH ANY IDEAS
THANKS KEV

http://www.youtube.com/watch?v=qZ3mnSB5jvo[youtube]